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re: solar vs solar steam ??
6 may 2001
jeffrey palmer  wrote:

>in an area with sufficient water supply, a simple yet effective cooling
>mechanism would be to have a garden soaker hose mounted at the peak of the
>roof. set to operated intermittantly, it would suck btus out of the attic
>as it cooled the roof via evaporation.

maybe, with a dark roof and no attic venting or insulation. at 3 pm and 86 f
(303 k) with w = 0.0133 (phila in july.) pw = 29.921/(1+0.62198/0.0133)
= 0.626 "hg, so dew point td = 9621/(17.863-ln(0.626))-460 = 65 f (18 c),
and sky temp ts = 303(0.711+.0056(18)+.000073(18^2)+0.013cos(15x14))^0.25
= 289 k (520 r), using an equation from page 158 of duffie and beckman's
1991 book, solar engineering of thermal processes. say 250 btu/h-ft^2 of
sun is falling, and the house is 70 f (530 r), and the roof has temp t (r),
where 250 = 0.1714x10^-8(t^4-520^4)+0.22(t-546)^1.33+(t-530)/r1, using
equation 2.23us from page 46 of krieder and rabl's 1994 book heating and
cooling of buildings, for upward turbulent convective heatflow from t to
86 f (546 r), with no wind, so t = 611 r (151 f.) with no attic venting
and r1 insulation, (151-70)/r1 = 81 btu/h-ft^2 flows into the living space.

with water, 250 = 0.1714x10^-8(t^4-520^4)+0.22(t-546)^1.33+(t-530)/r1+e.
e = 100(exp(17.863-9621/t)-0.626), so t = 563 r (103 f), and (103-70)/r1
= 33 btu/h-ft^2 flows into the living space.

but, if we paint the roof white with 80% solar reflectance and vent enough
to keep the attic air 90 f and add r40 floor insulation, 50 = 0.1714x10^-8
(t^4-520^4)+0.22(t-546)^1.33+0.22(t-550)^1.33+0.5, so t = 557 (97 f) and
(90-70)/r40 = 0.5 btu/h-f-ft^2 flows into the house. watering doesn't make
much difference. with an 8' ridge height and 4:12 pitch, a linear foot of
roof loses 24ft^2(0.22(557-550)^1.33) = 71 btu/h to the attic air, so we
need equal soffit and ridge areas a, where 71 = 16.6asqrt(8)(90-86.1)^1.5,
ie a = 0.2 ft^2/ft, eg a 2.5" slot.

>could the funny looks on a rooftop justify
>an energy savings in the suburbs?

no. your neighbors will shun you. nothing can justify
funny-looking roofs in the suburbs.


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